Inkjet inks which improve drop-velocity stability and prolong resistor life in inkjet pens

ABSTRACT

An inkjet ink composition and a method of printing with the ink composition comprising at least one colorant and an aqueous vehicle comprising at least one metal salt or metal-organic complex that forms a film on the thermal inkjet resistor surface after repeated energizing of the resistor.

FIELD OF INVENTION

The present invention relates to ink compositions suitable for thermalinkjet printing, and, more particularly, to ink compositions that arefilm forming and provide improved drop-velocity stability and prolongingresistor life in inkjet pens.

BACKGROUND OF INVENTION

The use of inkjet printing systems has grown dramatically in recentyears. This growth may be attributed to substantial improvements inprint resolution and overall print quality coupled with appreciablereduction in cost. Today's inkjet printers offer acceptable printquality for many commercial, business, and household applications atcosts fully an order of magnitude lower than comparable productsavailable just a few years ago. Notwithstanding their recent success,intensive research and development efforts continue toward improvinginkjet print quality, while further lowering cost to the consumer.

An inkjet image is formed when a precise pattern of dots is ejected froma drop-generating device known as a “printhead” onto a printing medium.The typical inkjet printhead has an array of precisely formed nozzleslocated on a nozzle plate and attached to an inkjet printhead substrate.The substrate incorporates an array of firing chambers that receiveliquid ink (colorants dissolved or dispersed in a solvent) through fluidcommunication with one or more ink reservoirs. Each chamber has athin-film resistor, known as a “firing resistor,” located opposite thenozzle so ink can collect between the firing resistor and the nozzle. Inparticular, each resistor element, which is typically a pad of aresistive material, measures about 35 μm×35 μm. The printhead is heldand protected by an outer packaging referred to as a print cartridge, ie., inkjet pen.

Upon energizing of a particular resistor element, a droplet of ink isexpelled through the nozzle toward the print medium, whether paper,transparent film or the like. The firing of ink droplets is typicallyunder the control of a microprocessor, the signals of which are conveyedby electrical traces to the resistor elements, thereby formingalphanumeric and other characters on the print medium.

The small length scale of the nozzles, typically 10 to 40 μm indiameter, require that the ink not clog the nozzles. Further, repeatedfirings of the resistor elements that must withstand many millions offirings over the life of the ink cartridge to be commercially practical,can result in fouling of the resistor elements and degrading penperformance. This build up of residue on the resistor elements is uniqueto thermal inkjet printers and is known as kogation and defined as thebuild-up of residue (koga) on the resistor surface.

Besides the problem of kogation, firing resistor surfaces aresusceptible to passivation layer damage by cavitation, contamination andmany other sources. Such passivation layer damage literally results inmicroscopic holes on the resistor surface which significantly shortenresistor life. Energizing of the firing resistor after hundreds ofmillions or even tens of billions of times can erode away the toppassivation layer, which is typically tantalum. This erosion may be froma combination of oxidation, chemical attack by the ink at hightemperatures, and cavitation.

Erosion of the top passivation layer can lead to the failure of theunderlying electrically insulating layers, causing the circuit whichprovides power to the resistor to short out. If the electricallyinsulating layers are not compromised, erosion can degrade drop velocitystability by adversely affecting the heat conduction properties of theresistor.

Minimizing drop-velocity variations between nozzles and within nozzlesis critical for accurate drop placement on paper. Drop placement errorsdegrade both text and image quality. The magnitudes of the placementerrors caused by velocity variations are dependent on pen-to-paperspacing and pen scanning speed relative to the paper. Therefore, asthermal inkjet printers become faster and print on a greater variety ofmedia, greater pen-to-paper distances will be needed and it will becomemore important to decrease drop velocity variations. Furthermore, dropplacement errors are more noticeable with small drop-volume pens; thesmaller drops cannot mask the errors.

Drop velocity variations are thought to be due to a combination oferratic drive bubble nucleation and variations in energies delivered toeach resistor. The former may be more important for velocity variationswithin a given nozzle. The latter may be more important for velocityvariations between nozzles and can be due to different resistancesthrough the electrical traces between the power supply and eachresistor. These parasitic resistances result in slightly differentamounts of power being delivered to each resistor. Erratic drive-bubblenucleation can be due to surface roughness or pits on the resistorsurface that provide low energy nucleation sites. Koga, a carbonaceousfilm formed from thermal decomposition of organic components in the ink,can especially contribute to surface roughness. Also, erratic bubblenucleation may be caused by sharp temperature gradients on the resistorsurface that may cause nucleation to occur first over the center hotspot of the surface of resistor as opposed to a uniform nucleation overa greater fraction of the resistor surface area. The problem of sharptemperature gradients is worse in small drop volume pens. In addition,sharp temperature gradients can lead to local high temperatures on theresistor. Higher resistor temperatures worsen kogation build up. Thisrough carbonaceous deposit provides many nucleation sites, leading toearly, erratic vapor-drive bubble formation, low drop velocity and dropweights.

Customer and profit demands require smaller drop volumes,color-laser-like ink permanence, and “permanent” print heads. Smallerdrop volumes give better spatial and chroma resolutions. However,passivation layer damage appears to be worse in smaller drop volumepens. In small drop volume pens each resistor must fire a greater numberof times to transfer the same amount of ink to the page. The greaternumber of firings required of the resistor results in more passivationlayer damage.

Reducing passivation layer damage by increasing the passivation layerthickness is typically not practical in high throughput printers.Resistors with thicker passivation layers require more energy to ejectan ink drop. However, most of this excess energy is retained as heatwithin the passivation layer and is not effectively transferred to theink. Therefore the power requirements are greater and more expensiveprinter components may be needed. Furthermore, this retained heat canbuild up in the thermal inkjet pens that would cause the pens tooverheat. Printing speeds would need to be reduced or elaborate coolingschemes employed to avoid the overheating.

The trend is towards longer print-head life, using pens with replaceableink supplies such as (but not limited to) off-axis ink reservoirs thatare connected to the pens by hoses and ink reservoirs that snap onto theprint head. Infrequent need for replacement of the print heads withprolonged resistor life reduces the cost and servicing required of thecustomer. High-speed, high-throughput photocopier-like products that maybe envisioned for the future will greatly increase ink usage and willmost likely greatly push further the demands on print-head life. Withhigher pen-to-paper relative speeds, high-throughput products will bemore sensitive to passivation layer damage induced drop velocityvariations.

Even though some kogation and/or passivation layer damage controlmethods in inkjet ink pens are known, all of them are either limited intheir effectiveness, are not economically feasible or have undesirableside effects for pens needing long resistor life. Thus, there is evenmore of a need to find a way to effectively deal with the problem ofpassivation layer damage on inkjet resistors.

Currently, tantalum is typically used as the material in the top coatfilm of the resistor. The metal is very hard and is resistant tocavitation damage. The metal has good chemical resistance. In spite ofthe beneficial properties of tantalum, the topcoat can erode afterrepeated firings many hundreds of millions or even tens of billions oftimes. In addition, defects in the tantalum can degrade the uniformnucleation properties of the surface and, as a consequence, diminishprint and image quality.

What is needed is a way of renewing the surface on the top of theresistor during the repeated firings of the resistor. The renewal of thesurface should be able to fill in pits and defects on the top coat andprovide a more uniform nucleation surface with a more uniformtemperature distribution during firing.

The film may not necessarily be hard if it can be renewed at asufficient rate. An analogy can be made with erosion of the shoreline.The tantalum is like a granite cliff that eventually erodes from theaction of the waves. The ceramic film of this invention is like a sandbeach. Though the sand is easily moved and eroded by the waves, thebeach will continue to exist as long as there is a sufficient supply ofsand from the neighboring beach or from a nearby river. In the case ofthe thermal inkjet resistor, a continual supply of metal ions for thefilm formation comes from the ink itself.

Due to the added passivation of the renewable surface, the tantalumlayer thickness can be thinned, minimizing the heat retained in thetantalum top coat. By providing a surface coating derived from theinteraction of the ink with the resistor, it will be possible tosubstitute the tantalum top coat with a less durable material includingsilicon, silicon oxide, silicon nitride, and silicon carbide.

It has been previously disclosed that film formation may benefitresistor life. In the background of a patent on chelates for kogationcontrol, Aoki and Koike disclose the possibility of beneficial filmformation as disclosed in Japanese Patent Application Laid-open No.56042684, with a substance used as a film-forming means in the ink toform a film on the surface of the heater. This surface film can relievethe shock to the surface that occurs at the time of generation andextinction of bubbles (cavitation). Substances that can be used asfilm-forming means can include metal-containing compounds such asorganic metal chelate compounds, metals of an organic acid, metallizeddyes and the like.

It has been previously disclosed that high amounts of aluminum salts canminimize black-to-color bleed and improve waterfastness. The addition of1 to 10 wt % of aluminum chloride and other multivalent salts tocationic-dye inks was patented by Stoffel for black-to-color bleedcontrol. Hackleman later patented a method for increasing waterfastnessby reacting anionic dye with aluminum chloride either in the media ordeposited on the media with a “fifth pen” with a 2 wt % aluminumchloride concentration.

At present, there is no patent literature concerning the addition ofsmall amounts of aluminum salts to ink comprised of anionic dyes,especially aluminum salt additions that lead to film formation on thethermal inkjet resistor surface. Here “small amounts” is meant below1000 parts-per-million (ppm). Furthermore there is no mention ofink-derived films improving thermal inkjet drop stability.

SUMMARY OF INVENTION

The present invention relates to a method of forming a metal oxide filmon a surface of a thermal inkjet resistor to reduce kogation and prolonginkjet pen life comprising firing the resistor at least one time toinkjet print an image on a medium with inkjet ink, wherein the inkcomprises:

at least one colorant; and an aqueous vehicle, the vehicle comprisingaluminum ion in an amount sufficient, when the composition is used in aninkjet pen, to form a protective thin layer on an outer layer of aresistor surface of the inkjet pen, the outer layer comprising arefractory metal, a noble metal, a silicon composition or mixturesthereof, the refractory or noble metal being selected from the groupconsisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, molybdenum, tungsten, gold, silver, platinum, silica, silicon,silicon nitride, silicon carbide and mixtures thereof.

The present invention also relates to a thermal inkjet printheadcomprising alumina-coated resistors.

Additionally, the present invention relates to a thermal inkjet inkcomprising:

at least one colorant; and an aqueous vehicle, the vehicle comprising

aluminum ion in an amount sufficient, when the composition is used in aninkjet pen, to form a protective thin layer on an outer layer of aresistor surface of the inkjet pen, the outer layer comprising arefractory metal, a noble metal, a silicon composition or mixturesthereof, the refractory or noble metal being selected from the groupconsisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, molybdenum, tungsten, gold, silver, platinum, silica, silicon,silicon nitride, silicon carbide and mixtures thereof.

Furthermore, the present invention relates to a method for inkjetprinting, said method comprising the step of ejecting ink, said inkcomprising: at least one colorant; and an aqueous vehicle, the vehiclecomprising at least one refractory or noble metal-reactive component inan amount sufficient, when the composition is used in an inkjet pen, toform a protective thin layer on an outer layer of a resistor surface ofthe inkjet pen, the outer layer comprising a refractory metal, a noblemetal, a silicon composition or mixtures thereof, the refractory ornoble metal being selected from the group consisting of titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, gold, silver, platinum, silica, silicon, silicon nitride,silicon carbide and mixtures thereof.

BRIEF DESCRIPTION OF DRAWINGS

The file of this patent contains at least one drawing executed in color.Copies of this patent with color drawing(s) will be provided by thePatent and Trademark Office upon request and payment of the necessaryfee.

FIG. 1 is the average drop velocity for a thermal inkjet pen plottingthe effect of increasing the number of resistor firings for an inkcontaining aluminum nitrate and an ink containing sodium nitrate as acontrol.

FIG. 2 is the pooled drop velocity variation within the nozzles for athermal inkjet pen plotting the effect of increasing the number ofresistor firings for an ink containing aluminum nitrate and an inkcontaining sodium nitrate as a control.

FIG. 3 is a photograph of a resistor of an inkjet pen fired up to 100million drops with an inkjet ink containing 50 ppm aluminum ions.

FIG. 4 is a photograph of a resistor of an inkjet pen fired up to 100million drops with an inkjet ink containing sodium nitrate in a molaramount equivalent to 50 ppm aluminum.

FIG. 5 is a photograph of a resistor of an inkjet pen fired up to 500thousand drops with an inkjet ink containing 50 ppm aluminum ions.

FIG. 6 is a photograph of a resistor of an inkjet pen fired up to 1million drops with an inkjet ink containing 50 ppm aluminum ions.

FIG. 7 is a photograph of a resistor of an inkjet pen fired up to 5million drops with an inkjet ink containing 50 ppm aluminum ions.

FIG. 8 is a photograph of a resistor of an inkjet pen fired up to 15million drops with an inkjet ink containing 50 ppm aluminum ions.

FIG. 9 is a photograph of a resistor of an inkjet pen fired up to 50million drops with an inkjet ink containing 50 ppm aluminum ions.

FIG. 10 is a photograph of a resistor of an inkjet pen fired up to 100million drops with an inkjet ink containing 50 ppm aluminum ions.

DETAILED DESCRIPTION OF INVENTION

The invention described herein is directed to inkjet inks for printinginkjet images using commercially available inkjet printers such as, forexample but not limited to, HP DeskJet® printers, manufactured byHewlett-Packard Company, of Palo Alto, Calif. The inks enable productionof near photographic images having little or no coalescence, excellentwaterfastness, and reduced dry time, on a range of print media, inparticular, for example but not limited to, plain white, 8½″×11″, 20 lb.weight, printer paper conventionally used for office and home computers.

As stated above in the background, something is needed to improve dropvelocity stability of inkjet resistors and something to provide arenewable resistor surface. Applicants have found that aluminum salts ininkjet inks create a beneficial film on the tantalum layer of the inkjetresistor.

Addition of aluminum nitrate to thermal inkjet ink at a level of atleast 50 ppm was found to unexpectedly improve pen performance. Relativeto the control ink without aluminum addition, the average dropvelocities are stabilized and the pooled standard deviations in the dropvelocity are reduced. These effects occurred early in the pen life ataround 5 million drops. The drop velocity changes with resistor“firings” up to the first 5 million drops per nozzle are believed to beassociated with burn-off of organic materials associated with penmanufacture rather than a build up of the effect of the aluminum salt.This burn-off effectively cleans the resistor surface.

Aluminum salt solutions must be carefully combined with thermal inkjetinks to avoid local supersaturation of the aluminum ions that may leadto precipitation of the aluminum. Not allowing enough time for thealuminum to mix and dissolve in the ink can lead to particulate build-upand clogging in the orifices in the thermal inkjet pen. Additions ofaluminum nitrate to ink vehicle without mixing forms a gel-likeprecipitate. When the ink vehicle was mixed well during drop-wiseaddition of the aluminum nitrate, no precipitate formed. It has beenfound that precipitation can be avoided by: (1) aging the inks to allowre-dissolution (2) insuring better mixing during aluminum nitratesolution addition or (3) using lower salt solution concentrations. Theavoidance of precipitation is an important manufacturing consideration.For example, addition of a strong base to raise the pH of a bulk inkcontaining aluminum salt may lead to local precipitation of aluminumhydroxide that would be slow to re-dissolve and could settle on thebottom of a storage tank.

In inkjet inks containing phosphate ions or phosphate esters, aluminumaddition can cause kogation and, even worse, cause drop velocityinstability. Thermal inkjet inks containing phosphate or phosphateesters are incompatible with the aluminum additions that are part ofthis invention.

Though it is difficult to determine the exact mechanism, speculation canbe made on why the addition of the aluminum nitrate salt to the thermalinkjet ink improves drop velocity uniformity and also reduces formationof carbonaceous koga.

The koga formed by the aluminum may have a role in evening out resistorvariations in a feedback mechanism that may offset over-energy.Over-energy, the energy in excess of that needed to fire a drop, can bean important contributor to kogation. Controlling over-energy to aminimum can be a strategy for reducing or eliminating kogation.Generally, however, some over-energy is required to compensate forresistor-to-resistor variations and pen-to-pen variations. Mostresistors are given more energy than necessary to assure that everyresistor has the minimum required energy.

With resistor firing of aluminum-spiked ink, two effects occur.Initially the organic residue from the manufacturing process burns awayand the aluminum salt forms a film. The boiling of the solvent leaves analumina behind as a precipitate. Where the resistor is hotter, the filmforms more quickly and provides a thermal-insulating layer, reducingheat transfer from the hot spots. By reducing maximum temperatures onthe surface of the resistors, kogation is reduced or eliminated.

The film formation by the thermal process gives a negative feedback“control” that evens out spatial variations on a given resistor andbetween resistors. In a sense the aluminum salt leads to the formationof beneficial film that does not lead to prenucleation or erratic dropejection. The mechanism may involve beneficial film properties includingachieving low surface energy (better nucleation) and a smooth,cavitation-resistant surface.

Because the benefit of aluminum nitrate reducing drop velocity variationoccurs very early, at around 5 million drops per nozzle, aluminaprecipitate can provide a better nucleation surface than theunconditioned tantalum by initially filling in defects on the tantalumsurface that could cause erratic nucleation.

Generally, the present invention relates to an inkjet ink compositionthat is film forming by repeated energizing of the resistor. This filmis beneficial for improving drop velocity stability and can prolongresistor life because the film can be continually refreshed duringrepeated energizing of the resistor.

In one embodiment, the present invention relates to a method of forminga metal oxide film on a surface of a thermal inkjet resistor to reducekogation and prolong inkjet pen life comprising firing the resistor atleast one time to inkjet print an image on a medium with inkjet ink,wherein the ink comprises: at least one colorant; and an aqueousvehicle, the vehicle comprising aluminum ion in an amount sufficient,when the composition is used in an inkjet pen, to form a protective thinlayer on an outer layer of a resistor surface of the inkjet pen, theouter layer comprising a refractory metal, a noble metal, a siliconcomposition or mixtures thereof.

In another embodiment, the present invention relates to a thermal inkjetprinthead comprising alumina-coated resistors. In a more preferredembodiment, the thermal inkjet printhead is a small volume thermalinkjet printhead. In another more preferred embodiment, the thermalinkjet printhead is an extended life thermal inkjet printhead.

In yet another more preferred embodiment of the thermal inkjetprinthead, the alumina-coated resistors are coated by a methodcomprising firing the resistor at least one time to inkjet print animage on a medium with inkjet ink, wherein the ink comprises: at leastone colorant; and an aqueous vehicle, the vehicle comprising aluminumion in an amount sufficient, when the ink is used in an inkjet pen, toform a protective thin layer on an outer layer of a resistor surface ofthe inkjet pen, the outer layer comprising a refractory metal, a noblemetal, a silicon composition or mixtures thereof.

In another embodiment, the present invention relates to a thermal inkjetink comprising: at least one colorant; and an aqueous vehicle, thevehicle comprising aluminum ion in an amount sufficient, when the ink isused in an inkjet pen, to form a protective thin layer on an outer layerof a resistor surface of the inkjet pen, the outer layer comprising arefractory metal, a noble metal, a silicon composition or mixturesthereof.

Additionally, the present invention relates to a method for inkjetprinting, said method comprising the step of ejecting ink, said inkcomprising: at least one colorant; and an aqueous vehicle, the vehiclecomprising aluminum ion in an amount sufficient, when the ink is used inan inkjet pen, to form a protective thin layer on an outer layer of aresistor surface of the inkjet pen, the outer layer comprising arefractory metal, a noble metal, a silicon composition or mixturesthereof.

In a more preferred embodiment of the above-described embodiments, therefractory metal, the noble metal, the silicon composition or mixturesthereof are selected from the group consisting of titanium, zirconium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,gold, silver, platinum, silica, silicon, silicon nitride, siliconcarbide and mixtures thereof.

In another more preferred embodiment of the above-described embodiments,the ink has an aluminum ion concentration range from 10 to 500 ppm. In ayet more preferred embodiment, the ink has an aluminum ion concentrationrange from 40 to 70 ppm.

In another more preferred embodiment of the above-described embodiments,the aluminum ions in the ink are obtained in the ink before the ink isinkjet printed by the step of adding aluminum to the ink, the aluminumbeing added to the ink in a form selected from aluminum salts, aluminumorganic chelates and aluminum metal.

In yet another more preferred embodiment of the above-describedembodiments, the aluminum coating on the thermal inkjet resistor surfaceis in a form selected from aluminum-oxyhydroxide, aluminum-hydroxide andaluminum-oxide.

In still another more preferred embodiment of the above-describedembodiments, the ink has a pH range from 3.5 to 5.5.

In another more preferred embodiment of the above-described embodiments,the outer layer of the resistor surface comprises tantalum.

In yet another more preferred embodiment of the above-describedembodiments, the inkjet pen has a minimum drop volume range of from 1 to10 picoliters. In a still more preferred embodiment, the inkjet pen hasa minimum drop volume range of from 3 to 6 picoliters.

In still another more preferred embodiment of the above-describedembodiments, the inkjet pen can be fired at least 50 million timeswithout being replaced. In a still more preferred embodiment, the inkjetpen can be fired at least 100 million times without being replaced.

In yet another more preferred embodiment of the above-describedembodiments, the ink in the inkjet pen is replaceable.

In another more preferred embodiment of the above-described embodiments,the at least one colorant is selected from a group consisting of a dyeand a pigment.

Surfactant

Surfactants suitably employed in the practice of the invention includeanionic and nonionic surfactants. Examples of anionic surfactantsinclude: Sulfonate surfactants such as Sulfosuccinates (Aerosol OT,A196; AY and GP, available from CYTEC) and Sulfonates (Aerosol DPOS-45,OS available from CYTEC; a Witconate C-50H available from WITCO; Dowfax8390 available from DOW); and Fluoro surfactants (Fluorad FC99Cavailable from 3M). Examples of nonionic surfactants include: Fluorosurfactants (Fluorad FC170C available from 3M); Alkoxylate surfactants(Tergitol series 15S-5, 15S-7, and 15S-9 available from Union Carbide);and Organosilicone surfactants (Silwet L-77 and L-76-9 available fromWITCO). These surfactants provide the necessary dot spread on plainpaper and special media, such as photobase glossy paper, for providingexcellent image quality.

Colorants

The inks made according to the present invention comprise at least onecolorant, preferably at least one dye. The amount of dye added to thevehicle in prior compositions and the inventive compositions tend to bea function of choice, and is largely dependent upon solubility of thedye in the vehicle and the color intensity of the dye. Typical amountsof dye are between about 0.1 wt % to about 10 wt % of ink composition,preferably, between about 0.1 and 5 wt %. In compositions of theinvention, the dye is preferably colored rather than black, although anyof the dyes used in inks for in-jet printers may be employed.Illustrative suitable dyes include Direct Blue 199 (available fromAvecia as Projet Cyan Special), Acid Blue 9; Direct Red 9, Direct Red227, Magenta 377 (available from Ilford AG, Rue de I'Iandustrie, CH-1700Fribourg, Switzerland), Acid Yellow 23, Direct Yellow 132, Direct Yellow86, Yellow 104 (Ilford AG), Direct Yellow 4 (BASF), Yellow PJY H-3RNA(Avecia), and Direct Yellow 50 (Avecia). More preferably, Direct Blue199, Magenta 377, and Ilford Yellow 104 are employed as the cyan,magenta, and the yellow colorants. Although in a preferred embodiment,the invention is directed to dye-based ink, addition of surface activephosphate esters would also provide benefit to pigment-based ink.

Other Ingredients

The inks of the present invention may optionally comprise componentssuch as buffers, metal chelators, and biocides, as are well known in theart of inkjet ink formulation.

Buffer

Buffers optionally employed in the practice of the invention to modulatepH can be organic-based biological buffers or inorganic buffers,preferably, organic-based. Further, the buffers employed should providea pH ranging from about 3 to about 9 in the practice of the invention,preferably about 4 to about 6 and most preferably from about 4 to about5. Examples of preferably-employed buffers include succinic acid,tris(hydroxymethyl)aminomethane, available from companies such asAldrich Chemical (Milwaukee, Wis.), 4-morpholineethanesulfonic acid(MES), and 4-morpholinepropanesulfonic acid (MOPS). Most preferably,succinic acid is employed in the practice of the invention.

The inks of the present invention optionally comprise 0 to about 1.5 wt% buffer. More preferably, the inks comprise from about 0.1 to about 0.5wt % buffer, with a concentration from about 0.1 to about 0.3 wt % beingthe most preferred.

Biocide

Any of the biocides commonly employed in inkjet inks may optionally beemployed in the practice of the invention, such as Nuosept 95, availablefrom Huls America (Piscataway, N.J.); Proxel GXL, available from Zeneca(Wilmington, Del.); and glutaraldehyde, available from Union CarbideCompany (Bound Brook, N.J.) under the trade designation Ucarcide 250.Proxel GXL is the preferred biocide.

INDUSTRIAL APPLICABILITY

The ink formulations are expected to find use in thermal inkjet printingapplications to increase dot gain while maintaining excellentcolor-to-color bleed alleviation, particularly when using photobaseglossy paper.

EXAMPLES Example 1

Ink was prepared according to Table 1.

TABLE 1 M377 Ilford dye Absorbance of 0.10 at 1:10000 dilution Pro-jet289 Absorbance of 0.12 at 1:10000 dilution 2-pyrrolidinone 8 wt %1,5-pentanediol 8.8 EHPD 1.9 Dowfax 8390 1 Succinic acid 1.8 Tergitol15-s-5 1. Tergitol 15-s-7. 0.6

The pH of the ink was adjusted to 4 using a solution of sodiumhydroxide. To one portion of the ink, aluminum nitrate was added as a10% aqueous metal salt solution to give an aluminum ion concentration of50 ppm. To a second portion of the ink, the control ink, an equivalentmolar amount of sodium nitrate was added. These two portions of ink wereused to fill thermal inkjet pens. Different sets of resistors on the penwere fired 0, 0.5, 1, 5, 15, 50 and 100 million times per resistor.Following the firings, drop velocities were measured to determine theaverage drop velocity and drop velocity variation for each nozzle.

The average drop velocity and pooled velocity standard deviations for athermal inkjet pen plotting the effect of increasing number of resistorfirings are shown in FIGS. 1 and 2, respectively, for an ink comprisingaluminum nitrate and an ink containing sodium nitrate as a control. Ascan be seen, the drop velocity stability is much better for the thermalinkjet pens with inkjet ink comprising aluminum nitrate salt. Thevelocity is much more constant over the range of firings and the dropvelocity variation is much less than for the pens containing the controlink with sodium nitrate.

Photographs of the resistor surfaces after firing 100 million drops areshown in FIGS. 3 and 4 for thermal inkjet pens with inkjet ink with thealuminum nitrate and the control ink, respectively. The aluminumaddition leads to the formation of a film on the resistor surface thatprogressively increases with the number of firings. Through surfaceanalysis the film is found to be composed principally of aluminum andoxygen heteroatoms and may be an aluminum oxide, aluminum oxyhydroxideor aluminum hydroxide phase. Little carbonaceous material is found inthe film. The film thickness is greater at center of the resistor wherethe surface has the highest. temperatures and is thinner near theperimeter of the resistor.

Photographs of the resistor surfaces of thermal inkjet pens using inkjetink with the aluminum nitrate after firing 500 thousand drops, 1 milliondrops, 5 million drops, 15 million drops, 50 million drops and 100million drops per nozzle are shown in FIGS. 5, 6, 7, 8, 9 and 10respectively.

Example 2

Organic acids are useful in thermal inkjet inks as buffers that helpstabilize the pH value of the ink. However, the type and concentrationof organic acid can affect whether an ink is film forming.

Ink was prepared according to Table 2.

TABLE 2 M377 Ilford dye Absorbance of 0.10 at 1:10000 dilution Pro-jet289 Absorbance of 0.12 at 1:10000 dilution 2-pyrrolidinone 8.75 wt %1,5-pentandediol 8. EHPD 1.9 Dowfax 8390 1 Tergitol 15-s-5 1. Tergitol15-s-7. 0.6

From this solution separate ink samples were prepared with the followingtypes and amounts of organic acids.

Ink was prepared according to Table 3.

TABLE 3 2-1 Malonic acid 1.61 wt % 2-2 Succinic acid 1.83 2-3 Glutaricacid 2.05 2-4 Adipic acid 2.28 2-5 Glycolic acid 1.17 2-62,2-Bis(hydroxymethyl)propanoic acid 2.09

The solutions above have different types of organic acids but haveapproximately equal molar concentrations. The pH of the ink was adjustedto 4 using a solution of sodium hydroxide. A 13.90% aqueous solution ofaluminum nitrate nonahydrate was added to each solution dropwise understirring to yield 50 ppm Al in each solution. Solutions were aged at 60C. for 24 h. Thermal inkjet pens were filled with the inks and fired upto 100 million times per resistor. Resistors were examined after firingfor film formation. Results are summarized in Table 4 for differentorganic acids. As can be seen organic acids with hydroxyl groups in analpha position do not lead to film formation for the formulations ofthis example. The alpha hydroxyl in combination with the carboxylategroup can act to chelate the aluminum ions and prevent their depositionas a film. In addition inks comprising malonic acid, a short-lengthdi-acid, and 2,2-bis(hydroxymethyl)propanoic acid did not lead to filmformation.

TABLE 4 2-1 Malonic acid No film formation 2-2 Succinic acid Filmformation 2-3 Glutaric acid Film formation 2-4 Adipic acid Filmformation 2-5 Glycolic acid No film formation 2-62,2-Bis(hydroxymethyl)propanoic acid No film formation

Example 3

The inventors investigated how sensitive the film formation is to thevehicle composition by varying the concentration of 2-pyrrolidinone inthe inks. Ink was prepared according to Table 5.

TABLE 5 M377 Ilford dye Absorbance of 0.10 at 1:10000 dilution Pro-jet289 Absorbance of 0.12 at 1:10000 dilution 2-pyrrolidinone 0, 1.3, 2.6,3.9, 5.2, 6.5, 7.8 and 9.0 wt % 1,5-pentandediol 8 EHPD 1.9 Dowfax 83901 Succinic acid 1.8 Tergitol 15-s-7. 1.5

The pH of the inks was adjusted to 4 using a solution of sodiumhydroxide. A 13.90% aqueous solution of aluminum nitrate nonahydrate wasadded to each solution dropwise under stirring to yield 50 ppm Al ineach solution. Solutions were aged at 60 C. for 24 h. Thermal inkjetpens were filled with the inks and fired up to 100 million times perresistor. Resistors were examined after firing for film formation. Filmsformed for all of the concentration of 2-pyrrolidinone investigatedincluding from the ink without 2-pyrrolidinone.

Example 4

Organic acids are useful in thermal inkjet inks as buffers that helpstabilize the pH value of the ink. However, the concentration of organicacid can affect whether an ink is film forming.

Ink was prepared according to Table 2 with the following amounts ofsuccinic acid: 0.5, 0.6, 0.7, 0.9, 1.1, 1.3, 1.6, 1.9, 2.3, 2.7, 3.3 and4.0%. The pH's of the inks were adjusted to 4 using a solution of sodiumhydroxide. A 13.90% aqueous solution of aluminum nitrate nonahydrate wasadded to each solution dropwise under stirring to yield 50 ppm Al ineach solution. Solutions were aged at 60 C. for 24 h. Thermal inkjetpens were filled with the inks and fired up to 100 million times perresistor. Resistors were examined after firing for film formation. Filmsformed for inks with succinic acid concentrations from 0.5 up to 2.3%and did not form for the inks with succinic acid concentrations of 2.7%or greater.

What is claimed is:
 1. A thermal ink jet printhead comprisingalumina-coated resistors wherein the alumina-coated resistors are coatedby a method comprising firing the resistor at least one time to inkjetprint an image on a medium with inkjet ink, wherein the ink comprises:at least one colorant; and an aqueous vehicle, the vehicle comprisingaluminum ion in an amount sufficient, when the ink is used in an inkjetpen, to form a protective thin layer on an outer layer of a resistorsurface of the inkjet pen, the outer layer comprising a refractorymetal, a noble metal, a silicon composition or mixtures thereof.
 2. Thethermal inkjet printhead of claim 1, wherein the refractory metal, thenoble metal, the silicon composition or mixtures thereof are selectedfrom the group consisting of titanium, zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, gold, silver,platinum, silica, silicon, silicon nitride, silicon carbide and mixturesthereof.
 3. The thermal inkjet printhead of claim 2, wherein the ink hasa pH range from 3.5 to 5.5.
 4. The thermal inkjet printhead of claim 2,wherein the outer layer comprises tantalum.
 5. The thermal inkjetprinthead of claim 2, wherein the inkjet pen has a minimum drop volumerange of from 1 to 10 picoliters.
 6. The thermal inkjet printhead ofclaim 5, wherein the inkjet pen has a minimum drop volume range of from3 to 6 picoliters.
 7. The thermal inkjet printhead of claim 2, whereinthe inkjet pen can be fired at least 50 million times without beingreplaced.
 8. The thermal inkjet printhead of claim 7, wherein the inkjetpen can be fired at least 100 million times without being replaced. 9.The thermal inkjet printhead of claim 2, wherein ink in the inkjet penis replaceable.
 10. The thermal inkjet printhead of claim 2, wherein theat least one colorant is selected from a group consisting of a dye and apigment.
 11. The thermal inkjet printhead of claim 7, wherein the inkhas an aluminum ion concentration range from 10 to 500 ppm.
 12. Thethermal inkjet printhead of claim 11, wherein the ink has an aluminumion concentration range from 40 to 70 ppm.
 13. The thermal inkjetprinthead of claim 7, wherein the aluminum ions in the ink are obtainedin the ink before the ink is inkjet printed by the step of addingaluminum to the ink, the aluminum being added to the ink in a formselected from aluminum salts, aluminum organic chelates and aluminummetal.
 14. The thermal inkjet printhead of claim 2, wherein the aluminumcoating on the thermal inkjet resistor surface is in a form selectedfrom aluminum-oxyhydroxide, aluminum-hydroxide and aluminum-oxide. 15.The thermal inkjet printhead of claim 1, wherein the printhead is asmall volume thermal inkjet printhead.
 16. The thermal inkjet printheadof claim 1, wherein the printhead is an extended life thermal inkjetprinthead.